A plurality of lower power inverters can be connected in parallel to form a higher power inverter. In order to connect a plurality of inverters to form a parallel inverter, the main problem to be solved is how to reduce the circulating current among the modules. It is necessary not only to achieve the increasing by integer multiples of the load-carrying capacity, but also to achieve the even distribution of the load, so that all of the inverters have the same MTBF (Mean Time Between Failures) in theory, thereby achieving the maximum of the parallel system MTBF. In order to achieve such an object, there are following solutions in the art.
In the first solution, it adopts a method of master-slave control in order to connect a plurality of inverters in parallel, that is, a control unit is used to control all of the power modules. All the power modules utilize the same driving signal of SPWM (sine pulse width modulation) to obtain substantially the same output, and its control block diagram is shown in FIG. 1. This control solution resolves the synchronization of the output voltages effectively, and adding the means of regulating bus voltage can achieve higher preciseness of even current. However, the disadvantage is the centralized form of control unit. A fault occurring in the control unit may cause the whole system paralyzed. Therefore, after the system is connected in parallel, the improvement of its MTBF is rather little because the fault bottleneck is presented.
To overcome the disadvantage of the first solution, the second solution is provided. In this solution, every inverter is provided with a control unit, but only one control unit is turned on at any time by way of intellective selection. If any fault occurs in the control unit, the system will jump to some other control unit automatically. Although the second solution resolves the problem of the fault bottleneck, the complexity and the cost of such a system are increased. Also, the switching of the driving wave is technically dangerous, which is likely to lead the damage of the power tube. Moreover, switching the control unit renders the jump of the amplitude or phase of the output voltage to some extent, and reduces the purity of the output voltage. Meanwhile, it may realize the parallel connection of only few power modules since the load-carrying capacity of the control circuit is limited. Another disadvantage of this solution is that a logic control unit must be added because of the necessity of controlling the switches concentratedly, and thus it not only increases the additional cost but also adds the new fault bottleneck.
The third solution is provided to reduce the fault rate of the master control unit and to prevent the danger brought by switching of the driving wave. In this solution, the parallel point is moved forward. The control block diagram of the improved parallel inverter is shown in FIG. 2, that is, the parallel point is moved forward to the output point of the voltage regulation. At any moment, only one of the selected switches K1˜Kn can turn on, that is, only one voltage regulating loop is selected to work, and other voltage loops are in a state of thermal backup.
In comparison with the second solution, the third solution can overcome not only the problem of fault bottleneck of the control unit, but also resolve the danger of the switching of the driving wave. And since the commonly shared units are fewer, the reliability is enhanced. However, the complexity of the system switching still exists. Switching may also render the jump of the amplitude and the phase of the output voltage to some extent, and the load-carrying capacity of the control circuit cannot be improved. Further, only few power modules can be connected in parallel. This is essentially still a kind of centralized control. This solution cannot overcome the problem of switching of centralized control switches as well, and should add a logic control unit. This increases the cost and the new fault bottleneck. Moreover, the user is likely to pull out the inverter being used as the master module due to the requirement of the hot plug, such that the problem caused by the master-slave switching is more serious.